1. Field of the invention
The invention relates to a method and an apparatus for finding the position of the mechanical axis of a vertebrate limb running in a longitudinal direction through a limb that is rotatably supported by its first end in a center of rotation that is not fixed and is not accessible for the mechanical determination of coordinates by means of a optical coordinate measurement device with an indicator to signal measurement point coordinates in one rotational position of the limb.
2. Description Of The Related Art
The mechanical axes of limbs, such as a human femur, must be known in order to insert prostheses/implants, such as a knee prosthesis, correctly. The resection that prepares the way for installation of the knee prosthesis on the femur must be oriented in accordance with the orientation of the femur's mechanical axis. Ordinarily the frontal regions of the femur are resected. However, at least one so-called dorsal and one ventral cut are also made at the femur, because the femoral component of knee prostheses is customarily U-shaped.
The exact position of the resection surfaces at the femur is of crucial significance for a long working life of the knee prosthesis. So far even an experienced surgeon finds the performance of this resection extremely demanding, because during the operation the standard bearing surfaces must be established according to the geometrical specifications of the knee prosthesis while taking into account the mechanical axis, and in some cases pathological displacements must also be corrected and allowances must be made for the position and action of the ligaments and muscles that are present.
Known sets of instruments for the implantation of knee prostheses comprise resection accessories in the form of cutting jigs that help the surgeon to guide a saw blade. For this purpose the cutting jigs are aligned as accurately as possible according to the mechanical axis of the femur, with the aid of various devices.
An apparatus of this kind to assist alignment of the cutting jig is disclosed in the document WO 00/00093. This apparatus consists substantially of a base part that can be releasably fixed in the distal region of the femur and a reference element that is connected to the base part so as to be pivotable and/or longitudinally displaceable, and that comprises a means of specifying a coordinate system. The reference element can be oriented so that its position is fixed precisely with respect to the femur, and the means for specifying the coordinate system are so configured that they can be used determine the orientation in which tools such as a cutting jig are attached.
A reference element of this kind is firmly connected to the femur and is aligned with the femur's mechanical axis. A locating rod is used to measure the mechanical axis. This locating rod is connected at its first end to the reference element, and its second end is directed toward the head of the femur, which is seated in the hip and through which the mechanical axis of the femur runs. In order to point the locating rod toward the head of the femur, the pelvic ridge is first palpated by the “two-finger” method. The disadvantage of “two-finger” palpation is that it is relatively inexact.
The patents U.S. Pat. No. 5,871,018 and U.S. Pat. No. 5,682,886 disclose methods for finding the position of the femoral mechanical axis. According to these procedures, in a first step the coordinates of the femur are measured, for example from an image obtained by computer-assisted tomography, and are stored in a computer. The stored data are then used to generate a three-dimensional computer model of the femur, and on the basis of this model the optimal coordinates are found for attaching to the bone a jig as well as the knee prosthesis that will eventually be installed. The basis for this operation is specification of the mechanical axis of the femur.
After such a simulation the patient's femur is fixed in position, and a recording device is used for the tactile sampling of individual points on the femoral surface in order to establish the orientation of the femur for the operation that is to be performed. This scanning of the bone requires either that the femur be exposed for much of its length, if possible as far as the hip joint, so that its surface is accessible to the recording device, or else that a kind of needle be used to pierce the skin and contact the bone as a sampling instrument. However, because any surgical intervention presents a risk to the patient, and insertions of a needle cause bleeding and hence an increased danger of infection of the associated bones, it is undesirable to carry out an additional surgical intervention in the hip region or to insert needles along the femur in order to find the position of the center of rotation. Furthermore, in this process the femur must be firmly fixed to the platform of the recording device, because otherwise the hip socket can be displaced during the fixation of the various instruments, so that after the femoral coordinates have been recorded, the cutting jig would be wrongly positioned.
The document FR 2 785 517 describes a method and an apparatus for detecting the center of rotation of the femoral head in the hip socket. For this purpose the femur is moved so that its head shifts within the socket, and the coordinates measured for various positions of the femur are recorded. As soon as shifting of the center of rotation of the femur occurs, a corresponding counterpressure is applied to the head of the femur, and this is taken into account for the specification of a point that is related to the position of the femur.
The objective of the present invention is to develop a method of this generic kind further, in such a way that it permits a rapid and simple acquisition of the information needed for the anatomically correct installation of an implant in dependence on the orientation of the mechanical axis of the limb, and also to disclose an arrangement for implementation of this method.